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3 years ago

A man takes 20 seconds to climb 5m up a ladder. He weighs 720N. Calculate the power he must deliver to do this.

Physics

1 answer:

Inessa05 [86]3 years ago

3 0

Answer:

180 W

Explanation:

The work done by the man against gravity is equal to its gain in gravitational potential energy:

$W=mg\Delta h$

where

(mg) = 720 N is the weight of the man

$\Delta h= 5 m$ is the change in height

Substituting,

$W=(720)(5)=3600 J$

The power he must deliver is given by

$P=\frac{W}{t}$

where

W = 3600 J

t = 20 s is the time taken

Substituting,

$P=\frac{3600}{20}=180 W$

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A ball is thrown nearly vertically upward from a point near the cornice of a tall building. It just misses the cornice on the wa

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Answer:

a) 48.5 ft/s

b) 36.5 ft

c) -80.3 ft/s

Explanation:

The equation of motion of the ball is :

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Where Vo is the initial velocity

If y(5s) = - 160 ft:

-160 ft = -16.1 ft/s^2 * (5 s)^2 + Vo*(5s)

Solving for Vo

Vo = (16.1*25- 160) ft / 5s = 48.5 ft/s

To answer this question we must first know when the velocity became zero, at this time is when the ball was at its highest point.

v(t) = -32.2 ft/s^2 * t + Vo

t = Vo/32.2ft/s^2 = 1.5 s

And now, the highest point which the ball reached is given by:

y(1.5s) = -16.1 ft/s^2 * (1.5)^2 + Vo*(1.5s)

y(1.5s) = 36.52 ft

We now need the time at which y(t') = -64 ft

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By means of the quadratic formula, we find that

t' = 4.00498 s ≈ 4 s

And the velocity at t = 4s is:

v(4s) = -32.2 ft/s^2 * 4s +48.5 ft/s = -80.3 ft/s

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Answer:

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at certain times the demand for electric energy is low and electric energy is used to pump water to a reservoir 45 m above the g

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Explanation:

Since the process is 70% efficiency, the power in output to the turbine can be written as

$P_{out} = 0.70 P_{in}$

where $P_{in}$ is the power in input.

The power in input can be written as

$P_{in} = \frac{W}{t}$

where

W is the work done in lifting the water

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The work done in lifting the water is given by

$W=mgh$

where

m is the mass of water

$g=9.8 m/s^2$ is the acceleration of gravity

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Combining the three equations together, we get:

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And solving for m, we find:

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Explanation:

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